From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A7A2390219 for ; Fri, 24 Apr 2026 11:53:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777031615; cv=none; b=fG0mRvW2pSW4UeWE9iPVC8XEsAoKWR0snBXP+M7Ub9Ywez8Yv/jMibtmKbncEvr0+PBAIF/rsuT8JQJJBeaUbJvjgKETUMhX4/XTXKNAMaWG+DXy4jFzP0NKBWcI6CHa0gKOaqrFRrLWkGEQXrpoppOomuhraWNQWRGuz+Nb1sY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777031615; c=relaxed/simple; bh=ElAcgQXNRHYhayu5c+VNdA15FV3TAK774nNECHjKGFg=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=WhtSls+5n2pnQel8Hp1q4j+R9tHlA0XuoFvt1/OAgw3D2UU0bIg9TDJ/yHEWPkKnXvX3jprTadra5rINpWOo2Bj5KXxI84AFKQChGfwV6lMrl2Ww7eS6ObhkXpCLksdDsTFTfVKdjgno7C3CzulLg+M+wuwijrJ4ApQDoRPZjUM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=XGHeCumi; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="XGHeCumi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2554C2BCB5; Fri, 24 Apr 2026 11:53:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1777031615; bh=ElAcgQXNRHYhayu5c+VNdA15FV3TAK774nNECHjKGFg=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=XGHeCumidfXtyQlwlHTFHq47UBB7t2V9ZZ1C1JtNFAG4K1i9dk4/jj2eukKjcckjf eeHHvUji2rdbDymzp+D3Sd9ljHbmwegU7IJQ3tnjAcEWa3Q+iIGdcZtEyRxN1d+tUV bYVz6MRaozMEEpt9keoFc7YISYEMa05veEKs9Eqw= Date: Fri, 24 Apr 2026 04:53:34 -0700 From: Andrew Morton To: "Barry Song (Xiaomi)" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , Xueyuan Chen , Kairui Song , Qi Zheng , Shakeel Butt , wangzicheng , Suren Baghdasaryan , Lei Liu , Matthew Wilcox , Axel Rasmussen , Yuanchu Xie , Wei Xu , Will Deacon Subject: Re: [PATCH] mm/mglru: Use folio_mark_accessed to replace folio_set_active in PF Message-Id: <20260424045334.ce9801ecd7a077bf3d5e4ff1@linux-foundation.org> In-Reply-To: <20260418120233.7162-1-baohua@kernel.org> References: <20260418120233.7162-1-baohua@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Sat, 18 Apr 2026 20:02:33 +0800 "Barry Song (Xiaomi)" wrote: > MGLRU gives high priority to folios mapped in page tables. > As a result, folio_set_active() is invoked for all folios > read during page faults. In practice, however, readahead > can bring in many folios that are never accessed via page > tables. > > A previous attempt by Lei Liu proposed introducing a separate > LRU for readahead[1] to make readahead pages easier to reclaim, > but that approach is likely over-engineered. > > Before commit 4d5d14a01e2c ("mm/mglru: rework workingset > protection"), folios with PG_active were always placed in > the youngest generation, leading to over-protection and > increased refaults. After that commit, PG_active folios > are placed in the second youngest generation, which is > still too optimistic given the presence of readahead. In > contrast, the classic active/inactive scheme is more > conservative. > > This patch switches to folio_mark_accessed(). If > folio_check_references() later detects referenced PTEs, > the folio will be promoted based on the reference flag > set by folio_mark_accessed(). > > The following uses a simple model to demonstrate why the current > code is not ideal. It runs fio-3.42 in a memcg, reading a file in a > strided pattern—4KB every 64KB—to simulate prefaulted pages that may > not be accessed. Are you able to suggest any workloads which might regress? And test for those? > Without the patch, we observed 12883855 file refaults and a very low > bandwidth of 58.5 MiB/s, because prefaulted but unused pages occupy > hot positions, continuously pushing out the real working set and > causing incorrect reclaim. With the patch, we observed 0 refaults > and bandwidth increased to 5078 MiB/s. Wow. And that isn't a crazy workload. > For those who want to try the model on x86, you will need the > following in arch/x86/include/asm/pgtable.h. > > #define arch_wants_old_prefaulted_pte arch_wants_old_prefaulted_pte > static inline bool arch_wants_old_prefaulted_pte(void) > { > return true; > } Can you propose a patch? We can at least toss it in there for testing while we think about it. > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -512,7 +512,7 @@ void folio_add_lru(struct folio *folio) > /* see the comment in lru_gen_folio_seq() */ > if (lru_gen_enabled() && !folio_test_unevictable(folio) && > lru_gen_in_fault() && !(current->flags & PF_MEMALLOC)) > - folio_set_active(folio); > + folio_mark_accessed(folio); > > folio_batch_add_and_move(folio, lru_add); > } lol, I was expecting something larger ;)